# std::ranges::uninitialized_fill_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
Call signature |  |
template< no-throw-forward-range I, class T > requires std::constructible_from<std::iter_value_t<I>, const T&> I uninitialized_fill_n( I first, std::iter_difference_t<I> count, const T& value ); |  | (since C++20) (constexpr since C++26)

Copies value to an uninitialized memory area first`+` `[`​0​`,` count`)` as if by return ranges::uninitialized_fill(std::counted_iterator(first, count),
std::default_sentinel, value).base();

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first | - | the beginning of the range of the elements to initialize
count | - | number of elements to construct
value | - | the value to construct the elements with

### Return value

As described above.

### Complexity

Linear in count.

### Exceptions

Any exception thrown on construction of the elements in the destination range.

### Notes

An implementation may improve the efficiency of the `ranges::uninitialized_fill_n`, e.g. by using ranges::fill_n, if the value type of the output range is TrivialType.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms

### Possible implementation

    struct uninitialized_fill_n_fn
    {
        template<no-throw-forward-range I, class T>
        requires std::constructible_from<std::iter_value_t<I>, const T&>
        I operator()(I first, std::iter_difference_t<I> n, const T& x) const
        {
            I rollback{first};
            try
            {
                for (; n-- > 0; ++first)
                    ranges::construct_at(std::addressof(*first), x);
                return first;
            }
            catch (...) // rollback: destroy constructed elements
            {
                for (; rollback != first; ++rollback)
                    ranges::destroy_at(std::addressof(*rollback));
                throw;
            }
        }
    };

    inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{};

### Example

Run this code

    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        constexpr int n{3};
        alignas(alignof(std::string)) char out[n * sizeof(std::string)];

        try
        {
            auto first{reinterpret_cast<std::string*>(out)};
            auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference");

            for (auto it{first}; it != last; ++it)
                std::cout << *it << '\n';

            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
    }

Output:

    cppreference
    cppreference
    cppreference

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed

### See also

ranges::uninitialized_fill (C++20) | copies an object to an uninitialized area of memory, defined by a range (algorithm function object)
uninitialized_fill_n | copies an object to an uninitialized area of memory, defined by a start and a count (function template)

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